I would appreciate some help in understanding roof design. The object, to correctly dimension principal or common rafters.

Can the equations for horizontal beam loading be used, working with the sloping loads projected onto the horizontal plane? I find no examples for this in Benson's books nor in the TFG Workbooks.

Further than this, I would like to go into the whole snow/wind loading formulae. I have looked at the American Wood Council's material but this uses span tables for 2" timbers only. Can someone reccomend tutorial material for this subject?

Thanks

I'm not qualified to answer but I think a rafter is designed as a simple beam with its horizontal span being used, not its diagonal run. I believe you then use the formulas for columns with axial and bending loads when it becomes a truss.

(I'm kinda piggybacking on your post hoping to get an answer myself )

You can use the horizontal dimension of the timber as long as you convert the dead load to account for this. Snow load is a projected load so no conversion is necessary.

Concerning your second question, if you are asking for a good design book try "Design of Wood Structures" by Breyer, Fridley and Cobeen. It doesn't concern timber framing directly, but will give you the basics of wood design that can be used in timber framing as well.

As always, nothing is a substitute for a properly trained and licensed engineer, someone I would encourage you to contact should you be looking to put your designs into practice.

Engineering is far more than span tables.

Thankyou Mr Ryan. Could I offer an exmple for complete clarity?

Rafters spaced at 2ft intervals span 14ft-2ins diagonally at 45deg. Assumed roof load is 40 lbs/sq-ft (projected horizontally) giving a total of 800lbs continuous on a 10ft horizontal span. The formulae suggest 3x4+1/2 (douglas), a section which would then be appropriate to span 14ft-2ins on the diagonal?

Thankyou for the book reccomendation.